At present, the known methods of obtaining an image of a stationary object with a pyroelectric infrared camera system are panning or orbiting and chopping. Panning or orbiting involves moving the camera, or lens, back and forth or in a circular motion. In most applications, a steady image is desired, therefore, image-motion compensation is required. This technique produces smeared images, particularly as the temperature contrast increases, and the additional size and weight of the mechanical apparatus required to pan or orbit the camera is a serious drawback in some applications. In its simplest form, chopping is accomplished by interrupting the beam of energy radiated by the source. This technique, without additional signal processing, produces flicker and the sensitivity is reduced.
A more sophisticated method of chopping referred to as image difference processing, has been developed. In this technique, radiation is chopped in synchronism with the frame, which generates frames having alternating scene polarity. Each frame is stored in memory, the following frame substracted from it, and the resultant signal displayed (after inverting the polarity of alternating frames). This technique improves the signal-to-noise ratio (compared to the normal chopping method) and reduces fixed pattern noise. However, since the scene radiation is received only 50% of the time, the sensitivity is reduced from that which can be obtained by the panning technique.